O-Phenylenediamine

o-Phenylenediamine (OPD) is an organic compound with the formula C₆H₄(NH₂)₂. This aromatic diamine is an important precursor to many heterocyclic compounds. OPD is a white compound although samples appear darker owing to oxidation by air. It is isomeric with m-phenylenediamine and p-phenylenediamine.

o-Phenylenediamine^[1]

Names
Preferred IUPAC name Benzene-1,2-diamine
Other names o-Phenylene diamine 1,2-Diaminobenzene 1,2-Phenylenediamine
Identifiers
CAS Number	95-54-5 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:34043 N
ChEMBL	ChEMBL70582 Y
ChemSpider	13837582 Y
ECHA InfoCard	100.002.210
EC Number	202-430-6
PubChem CID	7243
RTECS number	SS7875000
UNII	8B713N8Q0F N
UN number	1673
CompTox Dashboard (EPA)	DTXSID3025881
InChI InChI=1S/C6H8N2/c7-5-3-1-2-4-6(5)8/h1-4H,7-8H2 Y Key: GEYOCULIXLDCMW-UHFFFAOYSA-N Y InChI=1/C6H8N2/c7-5-3-1-2-4-6(5)8/h1-4H,7-8H2 Key: GEYOCULIXLDCMW-UHFFFAOYAR
SMILES Nc1ccccc1N
Properties
Chemical formula	C6H8N2
Molar mass	108.144 g·mol−1
Appearance	white solid
Density	1.031 g/cm3
Melting point	102 to 104 °C (216 to 219 °F; 375 to 377 K)
Boiling point	252 °C (486 °F; 525 K) Other sources: 256 to 258 °C (493 to 496 °F; 529 to 531 K)
Solubility in water	soluble in hot water
Acidity (pKa)	0.80 (doubly protonated form; 20 °C, H2O) 4.57 (conjugate acid; 20 °C, H2O)[2]
Magnetic susceptibility (χ)	−71.98·10−6 cm3/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H312, H317, H319, H332, H341, H351, H410
Precautionary statements	P201, P202, P261, P264, P270, P271, P272, P273, P280, P281, P301+P310, P302+P352, P304+P312, P304+P340, P305+P351+P338, P308+P313, P312, P321, P322, P330, P333+P313, P337+P313, P363, P391, P405, P501
NFPA 704 (fire diamond)	[3] 3 1 0
Flash point	156 °C (313 °F; 429 K)
Safety data sheet (SDS)	Oxford MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Preparation

Commonly, 2-nitrochlorobenzene is treated with ammonia to generate 2-nitroaniline, whose nitro group is then reduced:^[4]

ClC₆H₄NO₂ + 2 NH₃ → H₂NC₆H₄NO₂ + NH₄Cl

H₂NC₆H₄NO₂ + 3 H₂ → H₂NC₆H₄NH₂ + 2 H₂O

In the laboratory, the reduction of the nitroaniline is effected with zinc powder in ethanol, followed by purification of the diamine as the hydrochloride salt. Darkened impure samples can be purified by treatment of its aqueous solution with sodium dithionite and activated carbon.^[5]

Reactions and uses

o-Phenylenediamine condenses with ketones and aldehydes to give rise to various valuable products. Its reactions with formic acids to produce benzimidazole.^[6] Other carboxylic acids give 2-substituted benzimidazoles. The herbicides benomyl and fuberidazole are made in this manner. Thiophanate-methyl is another herbicide produced from o-phenylenediamine.^[4] Condensation with potassium ethylxanthate gives 2-mercaptobenzimidazole.^[7] With nitrous acid, o-phenylenediamine condenses to give benzotriazole, a corrosion inhibitor.^[8]

Quinoxalinedione may be prepared by condensation of o-phenylenediamine with dimethyl oxalate. Mercaptoimidazole are commonly used as antioxidants in rubber production, obtained by condensing xanthate esters. Condensation of substituted o-phenylenediamine with diketones yields various pharmaceuticals.^[9]

OPD is a ligand in coordination chemistry. Oxidation of metal-phenylenediamine complexes affords the diimine derivatives.^[10] OPD condenses with salicylaldehyde to give chelating Schiff base ligands.

It reacts with thionyl chloride to give 2,1,3-benzothiadiazole: C₆H₄(NH₂)₂ + 2 SOCl₂ → C₆H₄(N)₂S + SO₂ + 4HCl

Safety

With an LD₅₀ of 44 mg/L (in water), o-phenylenediamine is about 1000 times less toxic than the para-isomer. Anilines are typically handled as if they are carcinogenic. For many applications, OPD has been replaced by safer alternatives such as 3,3',5,5'-tetramethylbenzidine.^[11]